1. Field of the Invention
The present invention relates to an outer member of a multi-layer structure having an acoustic insulation layer and an image forming apparatus having a heat source inside the apparatus.
2. Description of the Related Art
In general, an image forming apparatus such as a copying machine uses parts such as a laser scanner, motor, solenoid, and clutch, which intermittently or stationarily operate at the image forming time. These parts generate noise in no small way at the operating time. Further, these parts often cause surrounding parts to resonate and generate the noise by not only the noise generated by them, but also the vibration generated by these parts. Further, though the image forming apparatus coveys a sheet such as a recording material, when the sheet is conveyed, the noise is also generated by rubbing, buckling, thrusting and the like of the sheet. Hence, to meet the situation, measures have been taken so far to reduce the noise by suppressing the noise generated inside the apparatus or preventing the noise generated inside the apparatus from leaking to the outside of the apparatus.
The measures to prevent the noise from leaking to the outside of the apparatus are usually taken by outer covers. The sound traveling to the outside of the apparatus includes the leaked sound leaking from the seams between the outer covers and louvers or the like, and the transmitted sound transmitting the outer covers. Usually, with respect to the leaked sound, there is a method for dealing with the situation by filling in the seams of the outer covers. On the other hands, with respect to the transmitted sound, it is known that an acoustic insulating effect of the outer cover of the multi-layer structure which alternately laminates solid layers and air layers is high. Sound waves generated inside the apparatus transmit through the air taken as an intermediary. However, when the solid layer such as the outer cover is interposed, impedance astronomically changes on the boundary surface between gas and one of liquid and solid. Hence, in that boundary surface, the energy of the sound is reflected approximately 100%, and hardly enters ahead of the boundary surface in the form of sound waves. In other words, the sound transmitting the outer cover which is audible outward is generated in such a way that the solid layer of the outer cover is vibrated by the mechanical force carried by the sound wave, and by that vibration, the air layer at the opposite side is vibrated. Here, when the transmitted sound transmits the solid layer, a transmission loss according to the type of the solid layer is generated. This transmission loss has nothing to do with the material of the solid layer, but is decided by its mass only. Hence, if the material high in density is used, the sound transmitting the solid layer is attenuated, and the energy which vibrates the air layer at the opposite side is also attenuated, so that the transmitted sound becomes small. Further, even when a porous acoustic insulation material is used for the solid layer, the transmitted sound becomes small. For this reason, according to the disclosure in Japanese Patent Application Laid-Open No. H06-348079, the outer cover of the multi-layer structure uses a sheet metal and resin for the inner wall and the outer wall to enhance the acoustic insulating effect, and uses the air layer and the acoustic insulation material for an intermediate layer between the two walls. Further, according to the disclosure in Japanese Patent Application Laid-Open No. H11-109976, there is a configuration in which, similarly as the acoustic insulation measures, a vacuum layer is formed between the outer wall and the inner wall so as to shut off the noise, and according to the disclosure in Japanese Patent Application Laid-Open No. H11-324707, there is a configuration in which the sound of the diesel engine is shut off by a sound-insulation cover material which is layer-formed of the acoustic insulation material and the sound-insulation material between a structural material (outer wall) and a diffusely reflecting material (inner wall).
As described above, since the outer cover of the multi-layer structure is high in sound-insulation effect, it is extremely effective as acoustic insulation measures. At the same time, however, the air layer and the acoustic insulation material serve as heat-insulation materials. While the image forming apparatus has a heating body such as a fixing device and a power source, an unnecessary heat must be discharged into the outside of the apparatus. Usually, the heat discharge is performed by forced heat discharge using a cooling fan and natural heat dissipation from the outer cover surface and the like. Since the outer cover of the multi-layer structure, as described above, performs the acoustic insulation as well as the heat insulation, the natural heat dissipation from the outer cover surface cannot be expected. To compensate for this heat discharge portion by the natural heat dissipation from the outer cover surface, it is conceivable to enhance a forced heat discharge capacity by using a cooling fan. To enhance the forced heat discharge capacity by using the cooling fan, there are methods for enlarging the size of the fan, increasing the number of rotations of the fan, enlarging the opening portion of the outer cover such as the louvers, and the like. However, in these methods, there is a possibility that the acoustic insulation effect by the above described outer cover is reduced. Further, though Japanese Utility Model Application Laid-Open No. 6-34955 discloses a configuration in which a heat conductive member is provided in a part of the outer cover, the provision of the heat conductive member in a part of the cover only is not enough for heat dissipation inside the apparatus, and the heat is liable to be accumulated at a position distant from the heat conductive member.